1. Field of the Invention
This invention generally relates to acoustic transducers for converting energy between electricity and sound and, in particular, to such transducers which use electromagnetism.
2. Statement of the Prior Art
Electromagnetic acoustic transducers are very common and have been widely used in perhaps all applications of acoustic transducers from audio reproduction to seismic mapping, to underwater detection, etc.. Generally, the use of electromagnetic versus piezoelectric transducers and the design parameters in any application are strongly application dependent and responsive to such considerations as desired frequency range, power level and operating environment. For this reason, transducers of various sizes, shapes and configurations have been developed for these various applications. Among the existing transducers are flexural and flextensional transducers. Such transducers are represented by U.S. Pat. Nos. 2,812,452; 2,920,307; 4,384,351; and 4,651,044.
U S. Pat. No. 2,812,452 is perhaps one of the earliest transducers using a slotted cylinder vibrating in the flexural mode. The slot runs longitudinally along the cylinder which is attached to a support along the portion of the cylinder diametrically opposed to the slot. In this manner, the halves of the cylinder on each side of the slot vibrate flexurally when driven by a piezoelectric material bonded to the inside of the metal cylinder. The patent boasts of a favorable power to weight ratio and may be coated with plastic or rubber for underwater use.
U.S. Pat. No. 2,920,307 provides a variable reluctance seismometer. The seismometer does not use an external carrier current and is sensitive to pressure or squeezing action as distinguished from displacement, velocity, or acceleration. The seismometer makes use of various cylindrical arrangements, employing a permanent magnet to establish a magnetic field in the cylinder and across an air gap. The air gap is formed as one or more longitudinal slots in the cylinder. A coil is magnetically coupled to the magnetic field of the permanent magnet to sense variations caused by changes in the thickness of the narrow air gap.
U.S. Pat. No. 4,384,351 discloses an electromagnetically driven flextensional transducer especially designed for use at greater water depths. A tubular shell having an oval cross-section is used with an electromagnetic element aligned with either the major or minor axis of the oval. The shell does not have to be pressure neutralized for greater depths because depth distortion does not impede the performance of the electromagnetic drive in the same manner as it would impede a piezoelectric drive.
U.S. Pat. No. 4,651,044 discloses a cylindrical piezoelectrically driven transducer operating in the flexural mode. The piezoelectric materials are cylindrically formed and bonded to the inside of the cylinder. In another form, the piezoelectric driver is mounted diametrically across the cylinder and orthogonally to the slot.
It is against this background that performance improvements are still desired for acoustic transducers in the power output below the frequency of 400 Hertz.